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YZ144 case mod w/Pics


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So I got a great deal on a 144 kit here on TT (thanks bkoz). Anyways, went to put it together today and it appears the cases have already been modified. The previous owner raced it in schoolboy so he may have did the 144 then switched it back.  The cylinder slid down into case and you can see where it looks worked on before. So, do I need to take any more off? the cylinder skirt slid right in so I assume I am good to go and I watched the EG video and it looks fine. But, this is the first time I have done this type before.

photo 1.jpg

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No that won't work, the cases are still stock. It's not the cylinder, it's the piston skirt that needs the clearance. When you look down the cylinder bore in your 2nd picture you can see the case protruding out into the bore area where the piston will hit it. You need to grind it back so nothing overhangs the cylinder.

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Alright. But I don't think the stock cases have grind marks, do they. And does the piston skirt extend below the cylinder skirt? There is no gasket on it in the pic so the cylinder will be raised more. I can Dremel it some more, but was hoping to not have to. I didn't think the cylinder would even slide down without case boring.

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The bottom of the skirt extends quite a bit down into the cases, that's why they need to be clearanced. Not all, but many of the cases have those grind marks from the factory from where they clean up blems from the casting process. You'll even find it in the port windows of stock cylinders.

 

If you want to check for yourself just install the piston and bolt on the cylinder, then slowly rotate the motor and you'll feel  it will stop when it hits the cases.

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its easy to do. you only need to remove from the front.

 

I like to put a piece of thin cardboard(file folder)in the front(1" to 1 1/2") between the crank and the cases, then seal that with some Plasticine and then duct tape the rest of the opening.

 

Then I put black marker(to make sure I am doing it even) on the surface I am going to grind off with my dremel (using a shop vac to suck up the filings). and repeat the process until I have removed enough.

 

when done pull off duct tape rotate crank backwards to remove cardboard and Plasticine and your done.

 

I think eric  gorr (thats where your cylinder was done) has instruction on his site.  

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